It is often necessary to use torque limiting devices in association with other machinery or equipment to prohibit the transfer of torques above some predetermined value so that damage to associated machinery or equipment does not occur. One typical application is an aircraft. Prime movers known as power drive units (PDU's), typically hydraulically powered, are utilized to move control surfaces of the aircraft as, for example, slats or flaps, between various positions. If movement of such an element is resisted as for example, by a mechanical jam in the drive line or some exterior influence exerting a force against the control surface preventing its movement, the power drive unit will tend to stall. Such units have high stall torques and as a consequence, if no steps are taken to prevent it, the stall torques can cause damage to the control surfaces, to the drive line, to the aircraft frame or all of the above.
A typical torque limiting device is illustrated in U.S. letters patent 4,030,578 issued Jun. 21, 1977 to Cacciola et al. In devices of this sort, a brake disc pack is employed and is compressed against a non-rotatable object as, for example, part of a housing. The compression is against the bias of a spring and the excess torque is taken up in the brake disc pack and grounded to the housing. Clearly, the coefficient of friction is a controlling factor in the amount of torque being taken up or absorbed within the brake disc pack.
Furthermore, the compression of the brake disc pack is obtained as a result of a ball ramp coupling that is interposed between the input to the torque limiter and the output thereof. In constructions such as that shown in the above-identified Cacciola et al patent, it is necessary that the balls in the ball ramp coupling be constructed of sufficient size so as to be capable of transmitting all of the torque up to the stalled torque level of the PDU to the brake disc pack to be absorbed thereby.
As a consequence of this type of construction, torque limiters have been fairly large and relatively unwieldy. They are unnecessarily heavy and thus impose a weight penalty in the environment in which they are most often used, namely, aircraft.
The present invention is directed to overcoming one or more of the above problems.